1. Field of the Invention
The present invention is directed to systems and methods for mapping and ablating body tissue of the interior regions of the heart. More particularly, this invention relates to catheters and methods for ablating cardiac tissues using a deflectable catheter having an irrigated ablation element for the treatment of cardiac arrhythmia, for example atrial fibrillation and ventricular tachycardia.
2. Description of the Prior Art
Abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid rhythm being referred to as a tachycardia. The present invention is concerned with the treatment of tachycardias which are frequently caused by the presence of an “arrhythmogenic site” or “accessory atrioventricular pathway” close to the endocardial surface of the chambers of the heart. The heart includes a number of normal pathways which are responsible for the propagation of electrical signals from the upper to the lower chambers necessary for performing normal systole and diastole function. The presence of an arrhythmogenic site or an accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions or tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including medications, implantable pacemakers/defibrillators, surgery and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by blocking or ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia.
Of particular interest to the present invention is radiofrequency (RF) ablation technique which has been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. RF catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined by diagnostic electrophysiology catheters which are connected to commercially available EP monitoring systems. After a mapping study, an ablation catheter is usually introduced to the target region inside the heart and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. RF energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of the arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated.
Atrial fibrillation (AF) is one type of cardiac arrhythmia believed to be the result of the simultaneous occurrence of multiple wavelets of functional re-entry of electrical impulses within the atria, resulting in a condition in which the transmission of electrical activity becomes so disorganized that the atria contracts irregularly. AF is a common arrhythmia associated with significant morbidity and mortality. A number of clinical conditions may arise from irregular cardiac functions and the resulting hemodynamic abnormalities associated with AF, including stroke, heart failure and other thromboembolic events. AF is a significant cause of cerebral stroke, wherein the fibrillating motion in the left atrium induces the formation of thrombus. A thromboembolism is subsequently dislodged into the left ventricle and enters the cerebral circulation where stroke may occur.
For many years, the only curative treatment for AF has been surgical, with extensive atrial incisions used to compartmentalize the atrial mass from the other cardiac tissue. The surgical “maze” procedure, as it is commonly known, includes, in the left atrium creating vertical incisions from the superior pulmonary veins, to the inferior pulmonary veins and ending in the mitral valve annulus, with an additional horizontal incision linking the upper ends of the vertical lines. It is believed that ectopic beats originating within or at the ostium of the pulmonary veins (PV) may be the source of paroxysmal and even persistent AF. As a result, a series of multiple point RF ablations to create linear patterns in the left atrium has been used to replicate surgical procedures in patients with paroxysmal or chronic AF. Although successful, there are instances in which this technique may leave gaps between lesions giving opportunities for reentrant circuits to reappear. Another challenge with using this approach is the high incidence of pulmonary vein (PV) stenosis.
Different energy sources have been utilized for electrically isolating the pulmonary veins from the atrium. These include RF, laser, microwave, cryoablation, light, and ultrasound energy. Each energy source has its advantages and disadvantages and PV isolation has been achieved to different degrees of success. Moreover several researchers have recently suggested that catheter ablation using ultrasound energy may reduce the incidence of PV stenosis.
Ventricular tachycardia (VT) is another abnormal heart rhythm that can be treated by catheter-based mapping and ablation systems. VT is an arrhythmia that results from electrical impulses arising from the ventricles instead of the heart's natural pacemaker known as the sinoatrial (SA) node. Due to the fact that electrical impulses are not conducting from the SA node to the ventricles, the ventricles contract abnormally very rapidly. As a result, the four chambers of the heart are less able to fill completely with blood between beats, and hence less blood is pumped to the remaining circulatory pathways in the body. Over time, VT can lead to heart failure or degenerate into ventricular fibrillation, which can result in myocardial infarction. The most common treatment for VT is antiarrhythmic medication, however if drugs are not effective, cardioversion may be recommended. Patients who still suffer from episodes of VT may require an implantable cardioverter defibrillator. Although RF catheter ablation as a treatment for VT has been performed, success has been limited particularly to those VT's due to ischemic heart disease. This may be due to the difficulty in achieving sufficient tissue penetration of RF energy to ablate arrhythmia circuit lying beneath the endocardial scar tissue. In addition, the ventricles are naturally much thicker than atrial tissues and creating transmural lesions is more challenging when utilizing conventional RF ablation techniques. Irrigated tip RF ablation catheters have recently been introduced to create deeper and larger lesions and have achieved the transmurality of the cardiac wall, however further improvements in performance is desirable. This may be due to the irregular endocardial surface of the ventricular trabeculum that may present a technical challenge for ablating electrodes of RF ablation catheters to make adequate tissue contact.
Cardiac ablation with ultrasound energy however, does not require good contact with the underlying tissue. Ultrasound energy propagates as a mechanical wave within the surrounding medium, and in the tissue the vibrating motion is converted into heat. Therefore the tissue does not need to be in intimate contact with the ultrasound transducer unlike in the case of RF ablating electrodes. At a designated zone within the tissue, the tissue at the target area can be heated to a sufficiently high temperature for ablation, while tissue surrounding the target area is subject to a lower intensity ultrasound energy and not damaged.
In order to produce thermal effect in the tissue, the ultrasound emitting member has a transducer. The piezoelectric nature of ultrasound transducers creates limitations for these materials. One major limitation is when the temperature of the transducer is elevated, the performance decreases steadily until no acoustic waves can be observed. One way to keep the operating temperature of this material cool is to irrigate the transducer. A cooling medium such as water is introduced or pumped through the catheter in order to dissipate heat in the housing and effect cooling of the transducer, including cooling of the external tissue surface to avoid unwanted tissue damage. Schaer et al in U.S. Pat. No. 6,522,930 describes a tissue ablation device in which the ablation element is covered by a tubular porous membrane that allows pressurized fluid to pass therethrough for ablatively coupling the ablation element to a tissue site. McLaughlin et al in U.S. Pat. No. 5,997,532 also describes an ablation catheter having a porous, non-conductive buffer layer over the tip electrode. In both the Schaer et al. and McLaughlin et al. catheters, the amount of irrigation fluid permeating out from the porous membrane or porous tip electrode to cool the tissue-ablation element interface may not be consistent due to its contact with the underlying tissue and therefore may produce a larger or smaller intended ablation area. This phenomenon can be more pronounced in irregular endocardial surfaces such as the ventricular trabeculae.
Thus, there still remains a need for a catheter-based system and method that provides improved ablation performance at the treatment location, especially in irregular endocardial surfaces.